Monitoring gaseous oxygen concentration

ABSTRACT

Apparatus for monitoring gaseous oxygen concentration that includes an oxygen sensor for providing an electrical sensor signal that varies as a function of oxygen concentration at the sensor. Processor circuitry compares oxygen concentration indicated by the sensor signal to at least one threshold level, and indicates when such oxygen concentration at the sensor departs from such threshold concentration level. The apparatus is calibrated by exposing the sensor to a calibration gas having an oxygen concentration equal to the desired threshold concentration level, and storing in the processor circuitry electrical indicia indicative of operating characteristics of the sensor at such threshold oxygen concentration level. When the apparatus is thereafter employed for monitoring a gas of undetermined oxygen concentration, the operating characteristics of the sensor reflected by the sensor output signal are compared to the prestored indicia for determining when oxygen concentration at the sensor crosses the threshold concentration level.

The present invention is directed to monitoring of gaseous oxygenconcentration, and more particularly to a method and apparatus forindicating when oxygen concentration in a monitored gas equals ordeparts from one or more threshold concentration levels.

BACKGROUND AND SUMMARY OF THE INVENTION

There are numerous applications in which it is desirable to indicatewhen gaseous oxygen concentration departs from--i.e., becomes eithergreater than or less than--one or more predetermined thresholdconcentration levels. For example, in the home health care environment,it is desirable to monitor the output of an oxygen concentrator todetermine when the output oxygen level decreases below a minimum desiredlimit, such as eighty-five percent oxygen. In other industrial andcommercial applications, it is desirable to maintain oxygenconcentration between preset lower and upper limits. Current devices formonitoring oxygen concentration are expensive and subject to tamperingat the application site. It is therefore a general object of the presentinvention to provide a method and apparatus for monitoring concentrationof oxygen in a test gas and indicating when such concentration departsfrom one or more preselected threshold levels, which are inexpensive tomanufacture and implement, which function reliably over an extendedoperating life, and/or which can be selectively reprogrammed at thefactory or in the field by properly trained and equipped personnel whileresisting tampering by unauthorized or untrained personnel.

Apparatus for monitoring gaseous oxygen concentration in accordance witha presently preferred embodiment of the invention comprises an oxygensensor for providing an electrical sensor signal that varies as afunction of oxygen concentration at the sensor. Processor circuitrycompares oxygen concentration indicated by the sensor signal to at leastone threshold level, and indicates when such oxygen concentration at thesensor departs from such threshold concentration level. The apparatus iscalibrated by exposing the sensor to a calibration gas having an oxygenconcentration equal to the desired threshold concentration level, andstoring in the processor circuitry electrical indicia indicative ofoperating characteristics of the sensor at such threshold oxygenconcentration level. When the apparatus is thereafter employed formonitoring a gas of undetermined oxygen concentration, the operatingcharacteristics of the sensor reflected by the sensor output signal arecompared to the prestored indicia for determining when oxygenconcentration at the sensor crosses the threshold concentration level.

In the preferred embodiment of the invention, the processor circuitry ismicroprocessor-based and may be programmed to detect a plurality ofoxygen concentration levels by sequentially exposing the sensor tocalibration gas at the various oxygen concentration levels, and storingelectrical indicia indicative of operation of the sensor at each suchcalibration level for later comparison to the sensor output during use.Such calibration operation preferably is performed at the time ofmanufacture, and the indicia of one or more calibration levels is storedin non-volatile memory for subsequent use in the field. The apparatuscircuitry and the sensor are mounted on a circuitboard assembly, withthe sensor and cardedge electrical contacts disposed along one edge ofthe board. The board may be plugged into calibration apparatus in whichthe contacts are connected to calibration control circuitry and thesensor is exposed to test gas at desired oxygen concentration through amanifold in the calibration apparatus. Recalibration in the field iseither not possible, or requires special knowledge and equipment onlypossessed by a trained technician.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with additional objects, features and advantagesthereof, will be best understood from the following description, theappended claims and the accompanying drawings in which:

FIG. 1 is a functional block diagram of apparatus for monitoring oxygenconcentration in accordance with one presently preferred embodiment ofthe invention;

FIG. 2 is a flowchart that illustrates operation of the processorcircuitry in the embodiment of FIG. 1;

FIGS. 3A and 3B are timing diagrams that illustrate output of the oxygensensor in the embodiment of FIG. 1; and

FIG. 4 is a perspective diagram that illustrates calibration of theapparatus of FIG. 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 illustrates apparatus 10 for monitoring gaseous oxygenconcentration in accordance with one presently preferred embodiment ofthe invention as comprising an oxygen concentration sensor 12 having asensor element 14 of zirconium oxide or other suitable solid-stateconstruction and a heater element 16 for raising the temperature ofsensor element 14 to a suitable level above ambient. Heater 16 isenergized by an amplifier 18 for raising the temperature of sensor 12 toa level of 400° C., for example, at which oxygen ions are mobile withinthe matrix of sensor element 14. Sensor element 14 thus provides ananalog electrical current signal that varies as a function of oxygenconcentration in the gas to which sensor 12 is exposed.

The sensor output signal is fed to a convertor 20, in which the analoginput signal is converted to a format suitable for digital processing.In the preferred embodiments of the invention, the analog input signalis converted to a periodic signal having periodicitycharacteristics--i.e., time duration and/or frequency--that vary as afunction of input current level. Such periodic signal indicative ofoxygen concentration is fed as an input to a processor 22. Processor 22is connected to a non-volatile memory 24 for selectively storing andretrieving calibration and measurement data. Processor 22 provides oneoutput to a driver 26 for energizing an LED 28, and another output to adriver 30 for activating an audible alarm or buzzer 32. Processor 22 mayalso receive an external control input 34. Processor 22 also enablesoperation of convertor when sensor output sampling is required, as willbe described.

Apparatus 10 illustrated in FIG. 1 is most preferably provided in theform of a printed circuitboard assembly 36, as shown in FIG. 4, havingsensor 12 mounted thereon. Processor 22 (FIG. 1) has an I/O bus 38 withlines connected to a series of card-edge contacts 38a-38n that extend inan array along one edge of assembly 36 adjacent to sensor 12. Bus 38 isalso connected within assembly 36 to an on-board DIP-switch socket 40(FIG. 1). Processor 22 and non-volatile memory 24 preferably areprovided in the form of a single integral microprocessor having on-boardnon-volatile memory for storing operating software as well as sensorcalibration indicia as will be described. Amplifier 18, drivers 26,30and convertor 20 may be of any suitable construction. All components ofFIG. 1 are on assembly 36, with the exception of input 35 in thepreferred embodiment, which is the calibration unit to be described.

To calibrate apparatus 10 in accordance with one feature of the presentinvention, assembly 36 (FIG. 4) is plugged into a calibration fixture 42that includes both a card-edge connector 44 for mating with contacts38a-38n, and a connector 46 for supplying a calibration gas atpredetermined oxygen concentration to sensor 12. With assembly 36 soinserted in calibration apparatus 42 and sensor 12 exposed to thecalibration gas, processor 22 is activated by operator input 34 (whichmay be within calibration apparatus 42) for monitoring the output signalfrom sensor element 14 and storing in memory 24 electronic indiciaindicative of operating characteristics of the sensor at the specificoxygen concentration of the calibration gas. This calibration proceduremay be repeated by sequentially inserting assembly 36 in othercalibration fixtures that expose sensor 12 to other calibrationconcentration levels, and sequentially storing in memory 24 indiciaindicative of sensor operation at each such oxygen concentration level.Thus, the operating circuitry is calibrated for the characteristics of aparticular sensor 12. Where multiple threshold levels are to bedetected, multiple LED's 28 may be provided, or a single LED may becontrolled based upon the relationship of the test gas to the multiplethresholds (as for instance inside or outside of a range). Thereafter,processor 22 monitors sensor 12 and energizes LED 28 and/or buzzer 32when oxygen concentration either exceeds or decreases below one of thecalibration levels.

FIG. 2 illustrates operation of apparatus 10 in an application formonitoring oxygen concentration and indicating when such concentrationdecreases below a single threshold level. Such an application issuitable, for example, in monitoring an oxygen concentrator in homehealth care applications as described above to indicate whenconcentrator output decreases below a desired minimum level such aseighty-five percent. Referring to FIG. 2, operation of apparatus 10 isinitialized at 50 by application of electrical power or resetting ofprocessor 22, and the prestored calibration indicia in memory 24 is readby processor 22. If input 34 (FIG. 1) indicates at 52 that apparatus 10is in a calibration mode of operation, sensor output calibration indiciais read by processor 22 at 54 and stored in memory 24. Processor 22 thenproceeds to the monitoring mode of operation 56, in which the output ofsensor element 14 is periodically sampled through convertor 20. Thisoperation is illustrated in FIGS. 3A and 3B.

FIG. 3A illustrates the output of convertor 20 when oxygen concentrationat sensor 12 is relatively high and above the minimum desired threshold.At time t1, processor 22 initializes operation at convertor 20, and theramp output of convertor 20 is monitored to a time t2 at which suchoutput exceeds a threshold T. FIG. 3B illustrates a similar conversionprocess at lower oxygen concentration, in which processor 22 againinitiates operation of convertor 20 at time t1', and monitors operationof convertor 20 to time t2' at which the output voltage again crossesthreshold T. It will be noted in FIGS. 3A and 3B that total timerequired for the convertor output to exceed threshold T is relativelyshort (t2-t1) at high oxygen concentration (FIG. 3A), but it isrelatively long (t2'-t1') at lower oxygen concentration (FIG. 3B). Thisconvertor operating time is a continuous monotonic function of oxygenconcentration at sensor 12. Processor 22 may thus determine when oxygenconcentration decreases below the desired minimum threshold, eighty-fivepercent in this example, when the time required for such conversionexceeds the conversion time determined and stored during the calibrationoperation. It will be appreciated, of course, that other methods ofsensor output conversion, such as pulsed frequency modulation atconstant duty cycle, or pulsed duty cycle modulation at constantfrequency, may also be employed.

Returning to FIG. 2, processor 22 monitors operation of convertor 20 asdescribed above, and compares the convertor output at 58 to thecalibration indicia prestored in memory 24 to determine whether oxygenconcentration is greater than or less than the calibration thresholdlevel. If monitored oxygen concentration is above the desired minimumthreshold level, a timer TLEDON is set equal to zero at 60, LED 28 isturned off, alarm 32 is turned off, and operation cycles to thebeginning 56 of the monitoring phase. Thus, as long as oxygenconcentration remains above the calibrated minimum desired level,operation continues in this loop. However, in the event that oxygenconcentration falls below the desired minimum level, operation proceedsto a step 62 at which the oxygen sensor/convertor output is examined todetermine if a probable sensor failure is indicated. This isaccomplished by comparing the oxygen concentration indicated by thesensor and convertor with the normal expected operating range. Forexample, in oxygen concentrator applications for home medical carediscussed herein by way of example, oxygen concentration would not beexpected to exceed a level of ninety-five percent, or decrease below alevel of twenty-one percent, which is the concentration of oxygen inair. Hence, if the output of sensor 12 and convertor 20 indicates anoxygen concentration greater than ninety-five percent or less thantwenty-one percent, this is interpreted by processor 22 as indicatingprobable failure at the sensor, such as a failure at heater element 16.In such an event 64, processor 22 flashes LED 28 through driver 26. Onthe other hand, if a sensor failure is not indicated, then LED 28 iscontinuously energized at 66. In either event, timer TLEDON isincremented at 68. The TLEDON timer is then examined at 70 to determinewhether the LED has been energized, either continuously or flashing forfifteen minutes. If so, buzzer 32 is energized at 72. In either event,operation is cycled to the monitoring step 56.

In accordance with a feature of the invention hereinabove described, thecalibration of the desired minimum and/or maximum oxygen concentrationthreshold(s) is accomplished at the factory at the time of apparatusmanufacture, and cannot be readily reprogrammed in the field. Thisfeature helps prevent accidental or intentional reprogramming of themonitor. However, a technician may selectively reprogram the monitor inthe field by inserting a DIP switch 48 in socket 40, and byappropriately setting the various elements in switch 48 while exposingsensor 12 to one or more known threshold concentration levels. Uponcompletion of this operation, DIP switch 48 is removed by the technicianso that apparatus 10 is again relatively tamperproof.

It is claimed:
 1. Apparatus for monitoring gaseous oxygen concentrationcomprising:an oxygen sensor for providing an electrical sensor signalthat varies as a function of oxygen concentration at said sensor,processor circuit means responsive to said electrical signal forcomparing oxygen concentration indicated by said signal to at least onethreshold level, means responsive to said processor circuit means forindicating when oxygen concentration at said sensor departs from saidthreshold level and means for calibrating said processor circuit meansto said at least one threshold level including means for removablyconnecting said calibrating means to said processor circuit means forcontrolling calibration of said processor circuit while said sensor isexposed to a gas having an oxygen concentration corresponding to saidthreshold level.
 2. The apparatus set forth in claim 1 wherein saidsensor and said processor circuit means are mounted together on acircuit board assembly and wherein said means for removably connectingsaid calibrating means to said processor circuit comprises a switchsocket on said assembly and a switch removably mounted on said socket.3. The apparatus set forth in claim 1 wherein said calibrating meanscomprises means for exposing said sensor to gas having an oxygenconcentration equal to said threshold level and means responsive to saidelectrical signal from said sensor for automatically storing calibrationindicia indicative of said sensor signal at said threshold concentrationlevel, said processor circuit means including means for retrieving saidstored indicia for comparison to said sensor signal to determine whenoxygen concentration indicated by said signal departs from saidthreshold level.
 4. The apparatus set forth in claim 3 wherein saidsensor and said processor circuit are mounted together on a circuitboard assembly having a card-edge contact means along one edge of saidassembly which is electrically connected to said processor circuitmeans.
 5. The apparatus set forth in claim 4 wherein said sensor isdisposed adjacent to said one edge, and wherein said means for removablyconnecting said calibrating means to said processor circuit meansincludes means for mating reception of said card-edge contact means onsaid calibrating means to control the calibration of said processingcircuit means and means for exposing said sensor to a calibration gashaving an oxygen concentration equal to said threshold level when thecircuit board assembly is inserted in said mating reception means.